Spray forming is a process by which a stream of molten metal is atomized by a gas stream impinging upon it. The resulting atomized droplets are then directed to a target by the gas stream, or the resulting atomized droplets are cooled to form a powder. Producing powders by typical prior spray forming methods results in a yield loss of 10-15%, and much of the loss is associated with powder being trapped in various areas of the apparatus rather than being delivered to the collection vessel during the process. In producing solid workpieces, known as preforms, typical prior spray forming methods result in a yield loss of 25-40%, and a significant portion of the loss is usually caused by over-spray and particles bouncing off the surface due to their angular impact relative to the normal of the preform surface. Various methods have been described to recover and reuse overspray powder, such as, for example, U.S. Pat. No. 5,649,993, but these are not wholly satisfactory.
Because many powders and preforms are susceptible to damage to their chemical structure by air and oxygen, they are often produced in a shield gas environment of nitrogen or argon. The flow of shield gas, however, must be turned off to allow workers to enter the chamber for cleanup, changeover and maintenance. Thus, any powder or preform remaining in the chamber becomes contaminated and unusable when air and oxygen enter the spray forming apparatus after the flow of shield gas is turned off.
Previously, gas streams or jets have been used to direct the path of the particles involved in the spray forming process. The gas streams typically consist of argon or nitrogen as the means of directing the particles, and heat is removed from the workpiece through conduction or convection.
Current processes for making powder metal products, particularly in materials used for critical aerospace applications, use a conventional gas atomizing process. In this process, high-pressure gas is directed at a molten metal stream to break it into smaller droplets. The droplets solidify as powder. For critical applications, the resultant powder is then blended with batches of powder from other small melts. The blend is screened to a small mesh size (325 mesh), canned and consolidated by extrusion into product suitable for manufacture into an aircraft component. This method of manufacture is not efficient because several small melts are required for blending, melts are made in conventional ceramic lined furnaces and hence result in oxide contamination, several powder handling operations offer opportunity for contamination, and many steps in the process make the production operation costly.
Heat transfer using non-equilibrium plasmas has heretofore been poorly understood and often incorrectly or inefficiently applied. There is a need in the art for methods and apparatus that improve the yield and quality of powders and preforms produced by spray forming. The present invention is directed to these, as well as other, important ends.